A steering gearbox receives a steering reaction force transmitted from a traveling wheel during a steering operation. For this reason, in order to absorb the steering reaction force, an elastic member such as a cylindrical rubber bush is interposed between a bolt and an attachment hole for attaching the steering gearbox to the vehicle frame.
However, in such a technique in which the steering gearbox is attached to the vehicle frame by interposing the elastic member therebetween, the steering gearbox moves by an elastic deformation amount of the elastic member before the rack shaft moves at the time a pinion engaging with a rack shaft starts to rotate, thereby causing a delay of the steering operation. In order to obtain a highly-precise steering performance by solving the delay of the steering operation, in recent years, there has been increased a steering gearbox having a rigid structure in which an iron sleeve is interposed therebetween instead of the elastic bush (Patent Document 1) or a steering gearbox having a rigid structure in which the bush formed of the elastic member is removed.
FIGS. 2 to 4 illustrate a main part of the steering gearbox of the known steering apparatus having a vehicle attachment structure formed in the rigid structure: FIG. 2 is a front view; FIG. 3 is a front view illustrating a state at the time of receiving the steering reaction force transmitted from a traveling wheel during a steering operation; and FIG. 4 is a view illustrating a state at the time of receiving the steering reaction force transmitted from the traveling wheel during the steering operation when viewed from allow IV shown in FIG. 2.
As shown in FIG. 2, a steering gearbox 10 is attached to a vehicle frame 2 such as a front sub-frame. In FIG. 2, the upward direction is referred to as an vehicular upward direction, and the downward direction is referred to as a vehicular downward direction. In FIG. 2, the transverse direction is referred to as a vehicular transverse direction. In FIG. 2, a direction perpendicular to a paper surface is referred to as a vehicular longitudinal direction.
A rack shaft 3 is slidably fitted to an inner circumference 12 of a hollow cylindrical portion 11 of the steering gearbox 10 in the transverse direction shown in FIG. 2. Tie rods 41 and 42 are connected to both ends of the rack shaft 3, and the tie rods 41 and 42 are connected to a traveling wheel through a knuckle arm (not shown).
A pinion insertion cylindrical boss 13 is integrally formed with the left end of the hollow cylindrical portion 11 so as to protrude upward from an outer circumference 15 of the hollow cylindrical portion 11. A pinion (not shown) engaging with the rack shaft 3 is formed in the lower end of a pinion shaft 5 inserted into the pinion insertion cylindrical boss 13. The upper end of the pinion shaft 5 is connected to the lower end of the steering shaft (not shown) connected to the steering wheel.
A rack guide insertion cylindrical boss 14 is integrally formed with the left end of the hollow cylindrical portion 11 so as to be adjacent to the pinion insertion cylindrical boss 13 and to protrude from the outer circumference 15 of the hollow cylindrical portion 11 in the longitudinal direction of the vehicle (a front-side direction which is perpendicular to a paper surface). A rack guide (not shown) is inserted into the rack guide insertion cylindrical boss 14 so as to guide the back surface (a side opposite to a rack tooth surface) of the rack shaft 3 using a roller etc. and to prevent the deformation of the rack shaft 3 caused by a reaction force upon engaging with the pinion, thereby allowing the rack shaft 3 to smoothly slide thereon.
When a user rotates a steering wheel (not shown), the pinion of the pinion shaft 5 rotates, and then the rack shaft 3 slidably moves left and right in accordance with the rotation of the pinion, thereby changing a steering angle of a traveling wheel.
Vehicle attachment boss portions 16 and 17 are formed in the left end (an pinion-side end) of the hollow cylindrical portion 11 and the right end (an counter-pinion-side end) of the hollow cylindrical portion 11, respectively, so as to protrude from the outer circumference 15 of the hollow cylindrical portion 11 in the longitudinal direction of the vehicle (the front-side direction perpendicular to a paper surface). Circular attachment holes 161 and 171 are formed in the vehicle attachment boss portions 16 and 17, respectively, in the vertical direction of the vehicle (the vertical direction shown in FIG. 2).
The steering gearbox 10 is attached to the vehicle frame 2 in a rigid structure (a rigid body structure) without interposing the bush formed of the elastic member by inserting bolts 163 and 173 into the attachment holes 161 and 171 and then by firmly fastening the bolts 163 and 173 to the vehicle frame 2.
When the steering apparatus having such a steering gearbox 10 of a rigid structure is steered, and then a steering reaction force F transmitted from a traveling wheel acts on the steering gearbox 10, the steering reaction force directly acts on the vehicle attachment boss portions 16 and 17.
As a result, as shown in FIG. 3, the steering gearbox 10 deforms about the attachment holes 161 and 171 of the vehicle attachment boss portions 16 and 17 to thereby deform in the vertical direction of the vehicle. Additionally, as shown in FIG. 4, the steering gearbox 10 deforms about the attachment holes 161 and 171 of the vehicle attachment boss portions 16 and 17 to thereby deform in the longitudinal direction of the vehicle.
In particular, differently from the vicinity of the vehicle attachment boss portion 16 in the left end (the pinion-side end) of the hollow cylindrical portion 11, the vicinity of the vehicle attachment boss portion 17 in the right end (the counter-pinion-side end) of the hollow cylindrical portion 11 is not provided with the pinion insertion cylindrical boss 13 or the rack guide insertion cylindrical boss 14. Accordingly, the rigidity in the vicinity of the vehicle attachment boss portion 17 in the counter-pinion-side end is small, and the deformation in the vicinity of the vehicle attachment boss portion 17 becomes large at the time the steering reaction force F occurs. As a result, a problem arises in that the steering stability deteriorates.
In this way, in order to suppress the deformation in the vicinity of the vehicle attachment boss portion 17 in the counter-pinion-side end, as shown in FIG. 2, the vehicle attachment boss portion 17 in the counter-pinion-side end is connected to the outer circumference 15 of the hollow cylindrical portion 11, and ribs 18a and 18b are formed in parallel to the central axial line of the hollow cylindrical portion 11. Additionally, in order to suppress the deformation in the vicinity of the vehicle attachment boss portion 16 in the pinion-side end, the vehicle attachment boss portion 16 in the pinion-side end is connected to the outer circumference 15 of the hollow cylindrical portion 11, and ribs 19a and 19b are formed in parallel to the central axial line of the hollow cylindrical portion 11. However, such parallel ribs 18a, 18b, 19a, and 19b do not have sufficient rigidity for suppressing the deformation of the vehicle attachment boss portions 16 and 17.    Patent Document 1: Japanese Patent Unexamined Publication No. JP-A-2001-80528